The present invention relates to the field of non-contact scanning techniques for detecting geometrical defects in articles, and more particularly to non-contact apparatus and method for detecting twist in articles such as pieces of lumber.
Non-contact systems for detecting geometrical and surface defects in articles, such as for detecting wane, holes, knots and roughness in pieces of lumber/timber and for generating grading and/or optimizing information based on such defects detection are known. An example of such a system is disclosed in U.S. Pat. No. 6,122,065 issued on Sep. 19, 2000 to Labbe et al. to the name of the present assignee, which system detects surface defects on a piece of lumber freely carried on a conveyer, in presence of some relative movement occurring between the piece of lumber and the conveyer surface while the inspection is performed. The system includes an inspection unit integrating an optical ranging subsystem using a laser and a camera for obtaining profile data through triangulation-based derivation techniques. Although the system of Labbe et al. is particularly efficient to detect roughness, cavities, wane, missing wood and altered wood on pieces of lumber that are conveyed at high speed, lumber grading based on the detection of these defects generally require the consideration of other deformation-related defects such as bow, cup, crook and twist.
Bow, cup and crook are all associated with two-dimensional edge profile deformations within planes that are respectively perpendicular/longitudinal, perpendicular/transverse and parallel/longitudinal to a main surface of the piece of lumber. Although the assessment of bow, cup and crook generally still involves human-based inspection in many mills, the two-dimensional deformation characteristics shared by bow, cup and crook are advantageously used for their measurements in a known automated method disclosed in U.S. Pat. No. 4,774,988 issued on Oct. 4, 1988 to Washburn et al., which method consists of applying successive, overlapping scans involving three (3) simultaneous measurements of edge distance with respect to a baseline while the piece of lumber is transported, determining for each scan any deviation from linearity of an interior position corresponding to a central one of the three (3) measurements, and computing an edge profile of the piece of lumber based on all deviations. Although being applicable to bow, cup or crook measurements, such method cannot be applied to the measurement of twist since it does not involve any edge profile.
In Canadian Patent Application no. 2,297,879 published on Aug. 3, 2001 to Carpentier, a non-contact method and apparatus for determining the shape of a workpiece in movement on a conveyor is disclosed, which uses a profiling unit including two or more laser diodes and a camera for capturing sets of profile images of the piece at predetermined intervals of time, wherein the last profile images of a given set are juxtaposed to the first profile images of a next set in an overlapping relationship, which profile images of overlapping sets as taken at different times are then correlated and transformed by an analyzing computer to compensate for wobbling movement of the workpiece that would otherwise skew the shape data. Although Carpentier teaches that such apparatus and method can be used to measure twist as well as bow and crook from the shape data obtained, the position of the overlapping images must be accurately measured and/or controlled to ensure that the transformation as operated by the analyzing computer does not generate cumulative error data components which may become significant in cases where the shape data is obtained over a large workpiece portion. It is difficult in practice to warrant that a shape inspecting system for use in combination with a high-speed workpiece conveyor be always calibrated in such a manner to maintain shape measurement errors within acceptable limits. Therefore, there is still a need for accurate, non-contact automated apparatus and method for measuring twist in articles such as pieces of lumber.
It is therefore a main object of the present invention to provide an apparatus and method for measuring twist in an article while being transported on a conveyor, which are substantially insensitive to relative movement between the article and the conveying surface, without suffering from accuracy limitations experienced with known prior art systems.
According to the above main object, from a broad aspect of the present invention, there is provided an apparatus for detecting twist along a reference axis in at least a portion of an article being carried on a conveyer in a conveying direction substantially parallel to the reference axis, the apparatus comprising a surface profile sensing unit mounted with respect to the conveyor and provided with a non-contact scanning device directing a pair of transverse scan line beams onto a surface of the article in spaced relationship in the conveying direction and repeatedly performing scans of corresponding simultaneously scanned pairs of spaced transverse areas of said surface while the article is conveyed, to generate profile data characterizing position of each transverse area in a reference system. The apparatus further comprises a data processor device for comparing with one another the profile data characterizing the respective position of the transverse areas of each scan to generate partial twist indicative data associated with each scan, and for summing the partial twist indicative data associated with all scans to obtain an indication of the twist in the article portion.
According to another broad aspect of the invention, there is provided a method for detecting twist along a reference axis in at least a portion of an article being carried on a conveyer in a conveying direction substantially parallel to the reference axis, the method comprising the steps of: i) directing a pair of transverse scan line beams onto a surface of the article in spaced relationship in the conveying direction while repeatedly performing scans of corresponding simultaneously scanned pairs of spaced transverse areas of said surface while the article is conveyed, to generate profile data characterizing position of each said transverse area in a reference system; ii) comparing with one another the profile data characterizing the respective position of the transverse areas of each scan to generate partial twist indicative data associated with each scan; and iii) summing the partial twist indicative data associated with all scans to obtain an indication of the twist in the article portion.